DFT calculations have identified the novel rearrangement shown here for decomposition of the Si derivative of the PETN explosive [PentaErythritol TetraNitrate (PETN), C(CH2ONO2)(4)] that explains the very dramatic increase in sensitivity observed experimentally. The critical difference is that Si-PETN allows a favorable five-coordinate transition state in which the new Si-O and C-O bonds form simultaneously, leading to a transition state barrier of 33 kcal/mol (it is 80 kcal/mol for PETN) and much lower than the normal O-NO2 bond fission observed in other energetic materials (similar to 40 kcal/mol). In addition this new mechanism is very exothermic (45 kcal/mol) leading to a Large net energy release at the very early stages of Si-PETN decomposition that facilitates a rapid temperature increase and expansion of the reaction zone.